Hitherto, a superconducting magnet apparatus including a superconducting coil, a helium tank, a radiation shield, a vacuum case, and a refrigerator is known as a superconducting magnet apparatus configured to generate a high magnetic field by using a superconducting coil in a superconducting state. The helium tank houses the superconducting coil and liquid helium. The radiation shield houses the helium tank. The vacuum case houses the radiation shield. The refrigerator includes a first cooling stage that is thermally connected to the radiation shield to cool the radiation shield and a second cooling stage for condensing helium gas in the helium tank. The helium tank includes a tank body that houses the superconducting coil, a refrigerator surrounding tube extending upward from the tank body and surrounding the refrigerator, and a communicating tube which extends upward from the tank body and through which an inside of the tank body and an outside communicate with each other. The vacuum case includes a first tubular portion that surrounds the refrigerator surrounding tube, and a second tubular portion that surrounds the communicating tube.
The superconducting coil in the superconducting magnet apparatus as described above is pre-cooled to a temperature at which the superconducting coil enters a superconducting state by a method described in Japanese Patent No. 5196781 (hereinafter referred to as “Patent Literature 1”), for example. In the pre-cooling method described in Patent Literature 1, liquid nitrogen is first supplied into the tank body through the communicating tube, for example, at normal temperature (for example, room temperature) and the superconducting coil is cooled by the liquid nitrogen to a first temperature (for example, 77 K). Then, the liquid helium is supplied into the tank body. By the liquid helium, the superconducting coil is cooled to a second temperature, that is, a temperature (for example, 4 K) at which the superconducting coil enters a superconducting state. Then, the tank body is filled with the liquid helium by an amount required for the superconducting coil to be immersed in the liquid helium. The superconducting magnet apparatus shifts to a steady-state operation after the superconducting coil is pre-cooled in this way.
In the method for pre-cooling the superconducting coil in the superconducting magnet apparatus described in Patent Literature 1, a large amount of liquid helium is consumed to cool the superconducting coil until the superconducting coil enters a superconducting state after the superconducting coil is cooled by the liquid nitrogen.